To understand this story you need to read Hasbro's line of original comics, specifically the chapters titled "Neigh Whatever You Want, Part 1 and Part 2."
You can find these chapters on the Pony Latino page: PonyLatino.com
That's right, the comics are also canonical in my fanfic :P
☆☆☆
☆ Detecting the Alteration ☆
"To summarize, you are telling me that Princess Cadence is going to marry some idiot named "Buck", but you are 100% sure that the wedding was going to be with Shining Armor, Twilight's older brother, but these events were apparently "modified"...?" Shadow said.
"Yes, that's what's happening, at that time I was still a prisoner on the Moon, but according to my sister, Princess Cadence became Shining Armor's girlfriend at one of the university dances, time passed and they decided to get married, but a moment ago my sister told me that the wedding would be with Cadence and Buck, and Cadence said that she never exchanged any words with Shining Armor in the past, something is wrong Shadow, it's like... the past has been altered." Luna said, a moment of silence invaded the place, it seemed that they had found the problem.
"The past... Shining Armor told me just now that he was a teacher at Canterlot University Academy." Shadow explained.
"So you talked to him... in short Shadow, there is a serious problem with the timeline." Luna said worriedly.
"Yes?, Explain to me." Shadow said.
"Well, Shining Armor is one of the generals of the Canterlot royal guard." Luna explained.
"... yes, we definitely have to solve this problem... Shining Armor said that when he was young he wanted to be part of the royal guard of Canterlot... but he said that was not for him... we have to hurry, he Changing something in the past is very dangerous, the change may be insignificant, but the consequences can be disastrous." Shadow explained upset.
"Sure... I have an idea, but we need to go to Cantetlot." Luna said determinedly.
"Okay, can you teleport us there?" Shadow asked.
"Yes, we better hurry." Luna answered.
They quickly teleported to Canterlot, specifically to the private library where Luna had found the different volumes of the book.
"Wow, this place is incredible, I've never seen these books before." Shadow said in amazement as he observed the large number of books he had in the room.
"This is the place where I found the books that I have been giving you, here it is full of old books, but there is one in particular that caught my attention once, one called "Universe, Space and Time", it talked about a investigation of the past, a spell that achieved time travel many years ago." Luna explained.
"Time travel... I see." Shadow said.
Luna was desperately searching for the book that contained the time travel spell.
"Here it is!, very good, we have to execute it as soon as possible, are you ready?" Luna asked with determination.
"Wait! You have to calm down a little Luna, I understand your desperation, but first I have to explain something to you, but before that I need you to teleport us back to my house." Shadow said.
"Your house?... is it okay." Luna said.
Luna and Shadow returned home in an instant.
"Now, I need you to come into this room." Shadow explained.
"An office?, Why do you want us to be here?" Luna asked confused.
"This is my personal "study", I have to explain some important things to you... temporal changes are very dangerous Princess Luna, if we change something from the past the present will also be affected, the vast majority of ponies who do not know Shining Armor or Princess Cadence will not suffer any significant changes, but the ponies that have been involved with those specific ponies will suffer alterations in their memories, the problem is that there are also memories that can be modified without the need to have a relationship with Shining Armor and Cadence ", ponies like Twilight, Celestia, Cadence, even your princess, will suffer a change in their memories." Shadow explained.
"I understand, but what is the need to make me enter this room?" Luna asked confused.
"You are very involved with Cadence's wedding, now I will ask you something, what happened the day you discovered that I was not of this dimension?" Shadow asked.
"That day I remember I was talking to my sister about Cadence's wedding... but I also remember that my sister didn't want to tell me who Cadence's fiancé would be... I got angry and decided to start work earlier, I started digging into the dreams of the ponies... and I found your door." Luna said.
"You started your job earlier than normal because your sister wouldn't tell you who Cadence's fiancé would be... but what would have happened if Celestia had told you it was Shining Armor?... you wouldn't have left angry, and Maybe you wouldn't have seen my dream... you would never have discovered me." Shadow explained.
"... it doesn't matter if the change is insignificant-" Luna said.
"The consequences will continue to be disastrous." Shadow concluded.
"I see your point, but what does this room have to do with this conversation?" Luna asked confused.
"Let's say I have learned to use chaos magic, this room exists on this existential plane... but it does not exist on this existential plane."
"... that's confusing." Luna said confused.
"If you leave the house and look for this room outside the house you will not find it, this room exists on an existential plane in which it exists... but it does not exist... what I want to get at, Luna, is that in this room You will be safe from temporal changes, this room is located in the "universal void", everything that enters here is isolated from its own dimensional and temporal changes, it is a bunker proof of Multiversal Alterations." Shadow explained.
"So in this room I won't lose my memories?" Luna asked.
"That's right, I'm not going to lose your memories." Shadow said.
"... Why?" Luna asked confused.
"... because I don't want to be alone again..." Shadow said sadly.
"... I see, it's okay, I'll stay here." Luna said.
"Very well... we have to start with the spell." Shadow said with determination.
Princess Luna prepared the spell quickly, she needed Shadow to be in the center of the room.
"Okay, I have the exact date, two weeks after high school students entered university, according to what my sister told me, before their memories were modified, Shining Armor and Cadence met on the first day of school in the university, and they became a couple when they danced at the King and Queen night of the prom, I will send you 3 weeks before the day of the prom." Luna explained.
"Okay, got it, I have to fix this problem before the dance." Shadow said determinedly.
"On this paper I wrote the spell to be able to go back in time, when you finish the mission you have to execute it and you will return to this timeline." Luna explained.
"Alright, everything ready, I have my saddlebag... that's all I need." Shadow said as he put the spell in the saddlebag.
"Okay, I'll get started, good luck Shadow." Luna said as she began to execute the spell, her horn lit up and Shadow's body began to levitate while a blue light circled around her body.
"3!...2!...1!...Ready!" Luna exclaimed with a countdown.
Shadow was gone, the room went completely silent, leaving Luna worried.
"...okay, everything will be fine..." Luna said worriedly.
At that moment a very loud sound began to sound through the studio door, something was happening outside.
"The present... is being rewritten." Luna said as she continued listening to the noise.
A noise similar to static, they could hear voices, the sound of busy cities, the sound of rivers, the sound of the entire population of Equestria constantly changing.
"...did he stop?..." Luna thought confused.
The silence was now absolute, nothing was happening outside, everything had calmed down.
"... this breeze... Shadow is coming back." Luna said as she watched how it began to get very windy inside the room, picking up the papers that she was lying around the room and disordering the books that were previously open.
Shadow appeared again, in the same place he had left a moment ago.
"Shadow!, What happened?, It didn't take you long to come back, Did something bad happen?" Luna asked confused.
"... don't worry princess... mission completed." Shadow said victoriously.
"You left 2 minutes ago." Luna explained.
"I know, and I'm sorry, I didn't explain it well before I left, when traveling to the past you can spend as much time as you want in the past, you will always return to the same moment you left, with a temporary change, in In this case it was 2 minutes, it depends on how long you go back in time." Shadow explained.
"I see, how long did it take you to complete the mission?" Luna asked.
"3 weeks, I almost didn't make it...technically I'm 3 weeks older now, and only 2 minutes passed, that's the magic of time." Shadow explained.
"Did you have any difficulties?" Luna asked.
"... let's just say Cadence and Shining Armor are a little shy." Shadow explained.
"... so... everything is resolved?" Luna asked worried.
"... yes, everything is solved, you can come home now, princess." Shadow said.
~Canterlot - 18 years in the past~
A light breeze invaded the place where Shadow was about to appear.
"Woaaaahhh *pum*... owh!, that hurt." Shadow exclaimed in pain.
Shadow appeared in what looked like a school bathroom stall.
"I think Luna didn't calculate the coordinates correctly, she appeared 1 meter above ground level." Shadow thought angrily.
Shadow was about to leave the bathroom, but he remembered something important.
"I can't go out looking like this, I need to be... bigger?... I need to adapt my body to a university one." Shadow thought worried.
He already had a perfect spell in mind for that task.
"It is very convenient to remember the Selformation spell perfectly, I suppose I will adapt my current appearance to that of a university pony, I will only increase my height to look taller and more robust." Shadow thought.
I wouldn't change my appearance, I just needed to look taller and more robust to blend in with the academy students.
"There... wow, it's strange seeing myself this way, now I see things from a higher height, my childish height limits me a lot... maybe I can get used to this." Shadow thought surprised.
Shadow could now leave the bathroom, ready to face the problem of the time alteration.
"AAAAAAHHHHHH!!!, there's a boy in the girls' bathroom!!!" A young mare exclaimed.
"... shit." Shadow thought upset.
Shadow quickly left the bathroom, apologizing countless times.
"Damn, I hope that wasn't Luna's joke." Shadow thought angrily.
Shadow began to walk through the hallways of the academy looking for Shining Armor or Cadence.
"How the hell will I find 2 ponies in an academy with more than 2,000 students?!" Shadow thought overwhelmed and stressed.
He continued walking for 15 minutes throughout the campus, until he finally found them.
"There's Cadence... and that has to be that idiot Buck." Shadow thought.
~1 hour before~
"So you can reveal memories using magic?" Shadow asked surprised.
"That's right, it's something that very few ponies can achieve, as you can see right now, I'm showing you the memory in which I talked to my sister about Cadence and Shining Armor's wedding, now look at this, this is a memory of Cadence , she herself designed it more than a year ago to teach my sister and me what the first interaction was between her and Shining Armor, as you can see here, Buck put his hoove into PoinDexter so that he would fall, behind him Shining Armor along with his friends, that's when Cadence helped him up, that was their first interaction." Princess Luna explained.
~ Current Events - After time travel ~
"Here comes Shining Armor, Buck has to put PoinDexter's hoove in so everyone falls and Cadence helps Shining." Shadow thought.
Shadow stared at the scene for a short moment.
"... wait, Buck didn't put his hoove in PoinDexter... this has to be the multiversal alteration!!!" Shadow thought frantically.
Shadow could only watch as Shining Armor and her friends slowly walked away from Buck, as if he were observing the scene in slow motion, a moment of frustration was unleashed, if she could not change this event, perhaps everything would be lost again. .
"Buck got distracted and didn't see PoinDexter, what the hell do I do?!!!" Shadow asked himself, trying to find a solution quickly.